Among pitch-type carbon fibers, carbon fibers using a mesophase pitch as a starting raw material have the advantage that products having an extremely high elastic modulus can be produced.
There have been even industrial products of pitch-type carbon fibers having an elastic modulus touching a level as high as 950 GPa which is almost equal to the theoretical elastic modulus of a graphite crystal in the direction of the axis A.
Pitch-type carbon fiber products with a tensile strength of a level of 3 to 4 GPa have also been commercially available. Also, pitch-type carbon fibers having the performance equal to that of polyacrylonitrile (PAN)-based carbon fibers representing the high strength carbon fibers have been available in recent years.
However, a composite material using the pitch-type carbon fiber has the problem that it has the compressive characteristics, especially, the compressive strength inferior to that of composite materials using the PAN-based carbon fibers. The practical properties, such as bending strength, of a composite material depend on the compressive strength of the carbon fiber. The pitch-type carbon fiber has a compressive strength lower than that of the PAN-based carbon fiber, which limits the application of the composite material using the pitch-type carbon fiber.
In order to solve this problem, Japanese Patent Application Laid-Open (JP-A) No. 2-14023 proposes a method in which a pitch containing 5 to 40% of an optically anisotropic phase and having a viscosity of 100 Pa.s that is conspicuously high for the spinning of the pitch is spun to produce a carbon fiber and thereby to improve the compressive strength. Also, Japanese Patent Application Laid-Open (JP-A) No. 3-816 discloses a method in which boron ions are injected into a pitch-type carbon fiber under vacuum to improve the compressive strength. These methods are extremely unique in operational conditions and require unpractical steps posing many drawbacks in the preparation of pitch-type carbon fibers industrially improved in the compressive strength.
Disclosed in Japanese Patent Application Laid-Open (JP-A) No.61-258024 is a method for producing a carbon fiber wherein a screen layer is attached to the end of an introduction port in the side of an outlet pore to pass a pitch through the screen layer and the outlet pore in this order thereby performing the spinning. However, industrial production using this method requires much labor and cost and also this method is yet insufficient to produce the compressive strength equal to that of the PAN-based carbon fiber.
In order to improve the compressive strength of a composite material while maintaining the rigidity of the carbon fiber, it is necessary to improve the compressive strength of the carbon fiber itself.